A muscle disorder is a disease that usually has a significant impact on the life of an individual. A muscle disorder can have either a genetic cause or a non-genetic cause. An important group of muscle diseases with a genetic cause are Becker Muscular Dystrophy (BMD) and Duchenne Muscular Dystrophy (DMD). These disorders are caused by defects in a gene for a muscle protein.
Becker Muscular Dystrophy and Duchenne Muscular Dystrophy are genetic muscular dystrophies with a relatively high incidence. In both Duchenne and Becker muscular dystrophy the muscle protein dystrophin is affected. In Duchenne dystrophin is absent, whereas in Becker some dystrophin is present but its production is most often not sufficient and/or the dystrophin present is abnormally formed. Both diseases are associated with recessive X-linked inheritance. DMD results from a frameshift mutation in the DMD gene. The frameshift in the DMD gene's transcript (mRNA) results in the production of a truncated non-functional dystrophin protein, resulting in progressive muscle wasting and weakness. BMD occurs as a mutation does not cause a frame-shift in the DMD transcript (mRNA). As in BMD some partly to largely functional dystrophin is present in contrast to DMD where dystrophin is absent, BMD has generally less severe symptoms then DMD. The onset of DMD is earlier than BMD. DMD usually manifests itself in early childhood, BMD in the teens or in early adulthood. The progression of BMD is slower and less predictable than DMD. Patients with BMD can survive into mid to late adulthood. Patients with DMD rarely survive beyond their thirties.
Dystrophin plays an important structural role in the muscle fiber, connecting the extracellular matrix and the cytoskeleton. The N-terminal region binds actin, whereas the C-terminal end is part of the dystrophin glycoprotein complex (DGC), which spans the sarcolemma. In the absence of dystrophin, mechanical stress leads to sarcolemmal ruptures, causing an uncontrolled influx of calcium into the muscle fiber interior, thereby triggering calcium-activated proteases and fiber necrosis.
For most genetic muscular dystrophies no clinically applicable and effective therapies are currently available. Exon skipping techniques are nowadays explored in order to combat genetic muscular dystrophies. Promising results have recently been reported by us and others on a genetic therapy aimed at restoring the reading frame of the dystrophin pre-mRNA in cells from the mdx mouse, the GRMD dog (reference 59) and DMD patients1-11. By the targeted skipping of a specific exon, a DMD phenotype (lacking dystrophin) is converted into a milder BMD phenotype (partly to largely functional dystrophin). The skipping of an exon is preferably induced by the binding of antisense oligoribonucleotides (AONs) targeting either one or both of the splice sites, or exon-internal sequences. Since an exon will only be included in the mRNA when both the splice sites are recognised by the spliceosome complex, splice sites have been considered obvious targets for AONs. More preferably, one or more AONs are used which are specific for at least part of one or more exonic sequences involved in correct splicing of the exon. Using exon-internal AONs specific for an exon 46 sequence, we were previously able to modulate the splicing pattern in cultured myotubes from two different DMD patients with an exon 45 deletion11. Following AON treatment, exon 46 was skipped, which resulted in a restored reading frame and the induction of dystrophin synthesis in at least 75% of the cells. We have recently shown that exon skipping can also efficiently be induced in human control and patient muscle cells for 39 different DMD exons using exon-internal AONs1, 2, 11-15.
Hence, exon skipping techniques applied on the dystrophin gene result in the generation of at least partially functional—albeit shorter—dystrophin protein in DMD patients. Since DMD is caused by a dysfunctional dystrophin protein, it would be expected that the symptoms of DMD are sufficiently alleviated once a DMD patient has been provided with functional dystrophin protein. However, the present invention provides the insight that, even though exon skipping techniques are capable of inducing dystrophin synthesis, the oligonucleotide used for exon skipping technique can be improved any further by incorporating an inosine and/or a nucleotide containing a base able to form a wobble base pair in said oligonucleotide.